The ultraviolet represents an extremely important region of the electromagnetic spectrum as it has a critical role in a wide variety of scientific, commercial, and government applications. For example, ultraviolet spectroscopy can be utilized to study planetary atmospheres to determine whether basic elements for life such as oxygen, nitrogen, and hydrogen are present. (See, e.g., Barth, C. A., Appl Optics 8, 1295, doi:10.1364/ ao.8.001295 (1969), the disclosure of which is incorporated herein by reference.) A UV spectrometer can also detect small quantities of complex organic molecules, (e.g., tholins), from the UV reflectivity of the surface of an icy moon providing information on how prebiotic chemistry takes place on extraterrestrial bodies. (See, e.g., Hendrix, A. R., et al., Icarus 206, 608-617, doi:10.1016/j.icarus.2009.11.007 (2010), the disclosure of which is incorporated herein by reference.) High sensitivity astronomical observations in the UV regime could also enable the observation of faint emission from the baryons that form the intergalactic medium, which likely represent 50% of the detectable baryonic mass in the universe. (See, e.g., Nicastro, F., et al., Science 319, 55-57, doi:DOI 10.1126/science.1151400 (2008), the disclosure of which is incorporated herein by reference.) Recent measurements enabled by the first all sky UV survey mission, GALEX, have uncovered a startling comet-like tail behind a red giant star that is streaking through space at nearly 300,000 miles per hour. This phenomenon is unique and can only be observed in the UV, and has now provided a means to characterize how stars can die and ultimately seed new solar systems through the shedding of carbon, oxygen, and other elements, (See, e.g., Martin, D. C. et al., Nature 448, 780-783, doi:10.1038/nature06003 (2007), the disclosure of which is incorporated herein by reference.) Extreme UV lithography is utilized to pattern the finest features of the latest generation of semiconductor devices. UV laser inspection and imaging is therefore critical to identify defects in the fabrication process to maximize yield and reduce cost in this highly competitive industry. (See, e.g., Chan, Y. D., et al. 76361D-76361D-76316, doi:10.1117/12.847371 (2010)UV, the disclosure of which is incorporated herein by reference.) UV imaging has also recently been used in medical imaging to study how caffeine affects calcium ionic pathways in the brain. (See, e.g., Tsai, T. D. & Barish, M. E., J Neurobiol 27, 252-265, doi:10.1002/neu.480270211 (1995), the disclosure of which is incorporated herein by reference.) Rockets produce significant UV emission due to the production of excited nitrogen oxide species in their plumes. (See, e.g., Levin, D. A., Proceedings of the SPIE 1764, 388-399, doi:10.1117/12.140868 (1993), the disclosure of which is incorporated herein by reference.) While infrared imaging is clearly an important anti-missile defense technology, UV can offer significant advantages even in this application due to the ability to observe even in direct sunlight using “solar-blind” imaging. Bite marks can be readily observed and identified in forensic investigations since human saliva (wet or dry) shines brightly under UV illumination. (See, e.g., West, M. H., et al., J Forensic Sc 32, 1204-1213 (1987), the disclosure of which is incorporated herein by reference.) Bruises are also evident for many days in UV after they have disappeared to the naked eye. As this only represents a small fraction of the real world applications of UV detectors, there is clearly strong motivation to have detectors with the highest possible sensitivity.
Unfortunately, despite this wide range of applications, scientific imaging in the ultraviolet is extremely difficult because the technology for sensing UV light is substantially limited by the quantum efficiency of available detectors and the transparency of optical coating materials. For example, many materials strongly absorb Near and Far UV light such that thicknesses of 20 nanometers or less are completely opaque. Thus, the UV throughput of an instrument is highly sensitive to impurities on, or contained in, any of the optical elements or the detector itself. The absorption depth of UV photons is also very short, making collection of photo-induced current difficult with traditional materials. Compounding this problem is that many important sources of UV light are faint, so maximizing detector sensitivity is critical to unlock the true potential of UV imaging for the above applications.
TABLE 1Performance of UV Detectors in Major Space Missions.Typical QuantumExample of CurrentEfficiencyUse in(155-300 nm)AstronomyCs2Te Microchannel~10% or lessGALEX Space TelescopePlatesSilicon CCD coated with~15-25% or lessCassini ISS, Hubble SpaceLumogenTelescope
Table 1, above, outlines some information for two examples of ultraviolet sensitive detectors that are currently in use in space missions along with their typical quantum efficiencies in the near and far UV. (See, e.g., Joseph, C. L., 3764, 246-253, doi:10.1117/12.371088 (1999) & Porco, C. C. et al., Space Science Reviews 115, 363-497, doi:10.1007/s11214-004-1456-7 (2004), the disclosures of which are incorporated herein by reference.) Although new classes of III-Nitride materials based on MBE or MOCVD grown GaN or AlGaN hold significant promise for future generations of UV detectors, they are expensive and extremely difficult to grow at sufficient quality to fabricate into imagers with low dark current and high responsivity. Silicon based charge coupled devices (CCDs), however, are based on the same fabrication technologies utilized in the semiconductor industry and are heavily used in commercial imaging applications such as digital cameras. This makes silicon CCDs cheap to manufacture with the added benefit that pixel design and layout are both extremely flexible. CMOS (Complementary Metal Oxide Semiconductor) based imagers are also widespread, and their use in commercial and scientific applications has been accelerating over the last few years due to the rapid progress of the overall CMOS industry. Unfortunately, the native oxide that naturally forms on silicon causes unfavorable distortions in silicon's electronic band structure in the near surface region. This leads to the capture of UV produced photoelectrons in surface traps and thus very poor response below 300 nm for silicon imagers. (See, e.g., Hoenk, M. E. et al., Appl Phys Lett 61, 1084, doi:10.1063/1.107675 (1992), the disclosure of which is incorporated herein by reference.) This can be overcome though a combination of techniques known as back illumination and back surface passivation.
Commercial methods, such as chemisorption and ion implant/laser anneal, do exist to passivate the back surface of silicon CCDs. (See, e.g., Lesser, M. P., 4139, 8-15, doi:10.1117/12.410521 (2000); Peckerar, M. C., et al., Appl Phys Lett 50, 1275, doi:10.1063/1.97882 (1987); & Lesser, M. P., 2198, 782-791, doi:10.1117/12.176777 (1994), the dislcosures of each of which are incorporated herein by reference.) However, there are limitations to these techniques in that they either 1: do not achieve 100% internal quantum efficiency, 2: have undesirably high dark current, or 3: are subject to hysteresis and stability issues due to adsorption of oxygen and other gases in the environment on the surface of the CCD. In contrast, surface passivation by delta-doping using silicon molecular beam epitaxy, enables precise control over the band structure at the CCD surface to get ideal Si reflection-limited response (see FIG. 1). (See, e.g., Burke, B. E. et al., 300, 41-50, doi:10.1007/1-4020-2527-0—6 (2004) & Nikzad, S., 2278, 138-146, doi:10.1117/12.180023 (1994), the disclosures of each of which are incorporated herein by reference.)
Even with ideal back surface passivation, the inherent reflectivity of silicon significantly limits the absolute detector quantum efficiency of silicon CCDs. This is illustrated by the dip in quantum efficiency to ˜25% near 280 nm in FIG. 1, which shows the measured and absolute quantum efficiency for a back-illuminated, delta-doped silicon CCD. Note that the data, once corrected for quantum yield, lies along the silicon transmittance curve. This indicates that the CCD is exhibiting reflection-limited response and 100% internal quantum efficiency. Data for an unmodified, front-illuminated CCD is shown for comparison purposes to illustrate the improvement in UV sensitivity that is achieved by the delta-doping and back-illumination processes. Based on these results, anti-reflection (AR) coatings should be utilized to maximize imaging performance. Modeling results predict that absolute quantum efficiencies of >50% should be achievable from in the near and far UV (100 nm-300 nm). (See, e.g., Hamden, E., et al., Appl Optics (2011), the disclosure of which is incorporated herein by reference.)
It should be noted that anti-reflective (AR) coatings are widely utilized for many detector systems (silicon, III-V, etc.) in the visible and infrared to improve absolute quantum efficiency. However, producing UV anti-reflection coatings is extremely challenging as the coatings must be extremely high quality: low in impurity/defect concentration to avoid UV absorption, and pinhole free dense to prevent humidity interaction with the imager surface. In addition, the index of refraction of silicon varies significantly over the UV, and therefore multiple different materials are required to cover the Near and Far UV effectively. This is especially important because even ideal materials have absorption cut offs that make them opaque in certain regions of the UV. In addition, a change of 2 nm or less can dramatically shift the peak anti-reflection performance or lead to dramatic changes in absorption cutoffs, especially in the far UV, making controllable and reproducible fabrication of ultrahigh performance AR coated silicon CCDs difficult. Recent modeling results by Hamden et al. have demonstrated that the target thicknesses for UV anti-reflective coatings range from 10 to 25 nm. (See, e.g., Hamden, E., et al., cited above.) When all of these constraints are combined (back-illumination, silicon band structure engineering/surface passivation, and AR coating deposition with sub-nanometer precision and accuracy), a series of sequential robust, nanoscale, surface-engineering processes are required to produce the best possible UV sensitive CCDs. Accordingly, a need exists to develop high-quality coatings to improve the optical properties of UV imagers.